Evaluation of diffusion barrier between lead-free solder systems and thermoelectric materials

The intermetallic compound SnTe rapidly formed at interfaces between p-type bismuth telluride (Bi _0.5Sb _1.5Te ₃) thermoelectric materials and lead-free solders. The intermetallic compound influences the mechanical properties of the joints and the reliability of the thermoelectric modules. Various lead-free solder alloys, Sn-3.5Ag, Sn-3Ag-0.5Cu, Sn-0.7Cu, and Sn-2.5Ag-2Ni, were used to investigate the interfacial reactions. The results thus obtained show that Ag and Cu preferentially diffused into the Te-rich phase in Bi _0.5Sb _1.5Te ₃, so layers of Ag-Te and Cu-Te compounds could not form an effective diffusion barrier. Electroless nickel-phosphorus was plated at the interfaces to serve as a diffusion barrier, and the (Cu,Ni) ₆Sn ₅ compound formed instead of SnTe. Furthermore, the intermetallic compound NiTe formed between nickel- phosphorus and Bi _0.5Sb _1.5Te ₃ and also served as a diffusion barrier. A plot of thickness as a function of annealing time yielded the growth kinetics of the intermetallic compounds in the thermoelectric material systems. The activation energy for the growth of the NiTe intermetallic compound is 111 kJ/mol.